Method of and apparatus for controlling vapor superheat temperatures



c. H. WOOLLEY ET AL 2,778,346 METHOD OF AND APPARATUS FOR CONTROLLINGVAPOR SUPERHEAT TEMPERATURES Jan. 22, 1957 Filed May 16, 1950 3Sheets-Sheet 1 27 2-4 1 p 35 5o4 Q 26 1 r965" E 305 /5 1 Z INVENTORSCharles H Min/leg] fidwz'n Dar/2am MIA- ATTO R N EY STE/4M FZOW 14M FLOWSTE/4M PRESSURE STEAMFZOW Jan. 22, 1957 c. H. wboLLEY ET AL METHOD OFAND APPARATUS FOR CONTROLLING VAPOR SUPERHEAT TEMPERATURES Filed May 16,1950 3 Sheets-Sheet 2 L11 ill 65/] 17 v c saw) EQ 9 LL m a E INVENTORSATTORNEY Jan. 22, 1957 C. H. WOOLLEY ET AL METHOD OF AND APPARATUS FORCONTROLLING VAPOR SUPERHEAT TEMPERATURES Filed May 16, 1950 STEAMTEMPERATURE "F 3 Sheets-Sheet 3 Ear/e H Maj/e9 ifdwm Durham ATTORNEYMETHOD OF AND APPARATUS FOR CONTROL- LING VAPOR SUPERHEAT TEMPERATURESCharles H. Woolley, Cranford, and Edwin Durham, Westfield, N. J.,assignors to The Babcock & Wilcox Company, Rockleigh, N. L, acorporation of New Jersey Application May 16, 1950, Serial No. 162,168

7 Claims. (Cl. 122-479) The present invention relates to the regulationof superheated vapor temperatures delivered by a vapor generating andsuperheating unit, and more particularly to an improved method of andapparatus for maintaining the vapor superheat temperature substantiallyuniform over a wide range of vapor output capacities.

In many vapor generating units it is desirable to maintain thetemperature of the superheated vapor substantially uniform over a widerange of vapor output capacities. Superheat temperature control isparticularly desirable in the generation of steam for the production ofelectrical energy in large central station power plants. In such plants,the upper limit of superheat temperature is governed by the materialsand construction of the turbine served by the steam. In the interests ofturbine efiiciency the temperature of the steam delivered to the turbineshould be maintained within close optimum limits throughout a wide rangeof capacities.

In the generation of and the superheating of the steam, a change in thesteam output requirements, with an accompanying change in heat input tothe water cooled furnace, will also change the temperature and volume ofthe furnace gases used in superheating the steam. In recent yearssuperheat steam temperatures have been closely regulated by means of agas by-pass arranged to divert heating gases around portions of thesuperheaters, or by attemperation of the superheated steam with steamcondensate or boiler feed water, or by use of a closed type heatexchanger arranged in the boiler circuit, such as a lower drumattemperator.

When utilizing steam attemperation or a gas by-pass the superheater isordinarily selected with sufiicient heat exchange surface to produce thedesired steam superheat temperature at an output capacity somewhat lessthan the maximum unit flow capacity, for example at 70 to 75% rating.Under such conditions the superheated steam temperature is thenmaintained at a uniform value between 75 and 100% of unit rating bymeans of the attemperator, or the heating gas by-pass. Ordinarily it isimpractical to provide attemperation or by-pass facilities for superheattemperature regulation for more than the approximate range of capacitiesindicated.

In accordance with the present invention an elongated vapor generatingfurnace having a convection type superheater adjacent the gas outlet endof the furnace is provided with a plurality of fuel inlets or burnersspaced at different positions from the superheater. The fuel input toone portion of the furnace is maintained substantially uniform over anupper range of vapor flow rates from the unit while the fuel input tothe remaining portion of the furnace is varied substantially inproportion to the actual flow rate from the unit. In the preferredembodiment of the invention hereinafter disclosed at least one fuelinlet or a row of inlets positioned closest to the furnace outlet isfired at a substantially uniform rate throughout a major portion of theoperating capacity range of the vapor generator. Other fuel inletsspaced further from the furnace outlet are fired at rates corre- "nitedStates Patent 2,778,346 Patented Jan. 22, 1957 sponding with changes inthe vapor generator capacity. As a result the superheat temperature ofthe vapor generated is maintained at a more uniform value over a greaterrange of output capacities than would be possible with a variation inall fuel inlet firing rates commensurate With the vapor generating loadrange. The operation of the vapor generating unit can be manuallyaccomplished in accordance with the method of the invention, or theregulation of the fuel input to the furnace can be attained by means ofautomatic controls hereinafter described.

The various features of novelty which characterize our invention arepointed out with particularity in the claims annexed to and forming apart of this specification. For a better understanding of the invention,its operating advantages and specific objects attained by its use,reference should be had to the accompanying drawings and descrip' tivematter in which we have illustrated and described our invention.

Of the drawings:

Fig. 1 is an elevation side view, partly in section, of a vaporgenerating unit constructed and arranged in accordance with theinvention;

Fig. 2 is a partial front view of the burner portion of the generatingunit shown in Fig. 1;

Fig. 3 is a schematic drawing of a control system suitable for theoperation of the unit shown in Fig. 1; and

Fig. 4 is a diagrammatic illustration of the operating results attainedby the present invention.

In the illustrated embodiment of the invention shown in Fig. l, thesteam generating and superheating unit includes a vertically elongatedfurnace to having a hopper bottom 11 and a heating gas outlet 12 in theupper portion thereof. The furnace is enclosed by fluid-cooled wallscontaining vapor generating tubes, where the tubes are connected intothe circulatory system of the steam generating portion of the unit. Thearch 13 below the gas outlet 12 is formed by a row of tubes 14 bent outof the plane of the furnace rear wall 15, with some of those tubesextended upwardly as a widely spaced row 16 to the furnace roof, whilethe remaining tubes 17 in the arch row are inclined rearwardly to mergeinto and extend upwardly in the plane of the furnace rear wall 15. Thetubes in the row 16 extend across the furnace gas outlet 12 in front ofsuperheating elements 18, which are positioned in the space between thetube row 16 and the tube extensions of the furnace rear wall 15.

Combustion gases are generated within the furnace 10 by the combustionof fuel, as hereinafter described, and pass through the furnace outlet12 and thence over the superheater elements 18. The gases in leaving thesuperheating elements 18 pass downwardly over additional banks of tubesof a superheater 2b and through the tubular elements of an economizer21. The tubes of the superheater 20 and the economizer 21 extendhorizontally, with the economizer extending from the outer side of thefurnace rear wall 15 to the outer wall 22 of the steam generator. Theeconomizer outlet conduits 23 extend upwardly in a vertical planeoutwardly spaced from the furnace rear wall 15 with the superheater 20positioned between the conduits 23 and the wall 22. The gas leaving thelower bank of the economizer is passed through a duct 2a to an airheater 25 before the gas is discharged through an induced draft fan (notshown) to the atmosphere. The duct 24 is formed with an inclined bottomand is provided with a plurality of gas flow control dampers 26controlling the flow of gas between the superheater elements 18 and theair heater. The tubes 23 form part of a partition wall 27 extending fromthe level of the upper end of the superheater 20 to the dampers 26. Withthis construction, the amount of gas flow through the superheater 2t)can be regulated, and

thus the temperature of the superheated steam delivered by the unit.Some of the heat in the gases passed through the by-pass path 28 definedbetween the partition wall 27 and the rear wall of the furnace, will beabsorbed by the portions of the economizer 21 extending across theby-pass path.

Fuel is supplied to the furnace 10 through burners installed in thefront wall thereof. As shown in Figs. 1 and 2, the burners are arrangedin three rows, the lower row 39C positioned superadjacent the furnacehopper bottom 11, while the intermediate and upper burner rows, B and Arespectively, are vertically spaced therefrom. Any number of spaced rowsof burners, i. e. two or more, may be used in accordance with thepresent invention. As shown in Fig. 2, each burner row consists of 2burners, although any number of burners may be installed in each row,depending upon the Width of the furnace ltl and the amount of fuel to bedelivered thereto. Each row of burners is supplied with pulverized coalfrom an individual coal pulverizer 31, with each pulverizer connected toburners of a corresponding burner row by separate burner lines.Pulverizer 31C is shown in Fig. 1 with its air inlet duct, and burnerline connections with the burner row 30C. Pulverizers 31A and 31B (notshown) are the same size and capacity as pulverizer 31C, are providedwith similarair inlet ducts, and are connected with the burner rows 30Aand 30B, respectively.

A forced draft fan 32 is arranged to deliver a controlled amount of air,at the desired pressure, to the air heater 25 with the preheatedcombination air therefrom passing into a duct 33. The duct 33 isconveniently arranged along one side of the furnace and is provided withan upright extension 34 located adjlacent a casing 35 enclosing theburners 30. The casing 35 is provided with vertically spaced horizontalpartitions separating the easing into a plurality of air chambers 36A,36B, 36C and 37. The chambers 36A, 36B and 36C enclose and open to theburner rows 30A, 30B and 30C respectively, and are each in communicationwith the air duct extension 34 through a dampered conduit connection38A, 38B and 38C, respectively. The lower end of the air duct extension34 opens into the chamber 37 at the lower end of the casing 35, fromwhich individual primary air ducts 40A, 40B and 40C lead to the faninlet connection of each of the pulverizers 31A, 31B and 310respectively. Each of the primary air ducts is provided with a damperedtempering air inlet connection, such as 41C, and a primary air flowcontrol damper, such as 420. With the described pulverizer air flowconnections, the temperature and amount of air delivered to eachpulverizer can be regulated to control the pulverized coal output of thepulverizer and the amount of fuel delivered to each row of burners. Thecontrol of pulverized coal flow from a unit pulverizer by means of aregulation of the primary air flow thereto is well known in the art.Thus, regulation of primary air flow accomplishes a correspondingregulation of fuel flow to the burners. The flow of secondary combustionair to each burner row can also be regulated for optimum combustionconditions.

In the control diagram shown in Fig. 3, the forced draft fan 32 (Fig. 1)is controlled to provide an optimum amount of combustion air to theburners 30 for the combustion of fuel within the furnace 10. The fan 32is provided with both speed control and damper regulations (not shown),both of which are actuated in response to a steam flow-air flow type ofcontrol system. The fan speed is regulated by flow of fluid through thevalves 45 and 46 to a hydraulic coupling (not shown) on the fan drive.Since fan speed changes will not only change the volume of airdelivered, but also the air pressure, the extent of air flow regulationby fan speed is limited. Any increase or decrease in fan air flow,beyond that obtainable by changes in fan speed, is obtained by the useof vanes in the fan inlet, as positioned by a power piston 47. This typeof control system is well known and consists of a steam flow-air flowratio controller 48, where a change in the steam flow from the steamgenerating unit causes a proportional change in the position of apneumatic pilot valve 50. The valve transmits an impulse through aStandatrol 51 and an averaging relay 52 to both the piston 47 whichpositions the vanes in the fan inlet, and to the control Valves 45 and46 regulating the flow of fluid through the variable speed hydraulic fancoupling. The cilibrating relay 53 regulates the transmittal of controlimpulse to the power piston, while the valves 45 and 46 regulate fanspeed through a range of fan speeds in accordance with the adjustment ofa differential relay 54 correlated with a hydraulic tachometer 55 andacting through an accelerating relay 56, so that the piston 47 willbecome operative below the selected fan speed operating range. Theregulation of the fan 32 in accordance with the steam flow-air flowcontroller 48 controls the total amount of combustion air delivered,with the fuel, to the furnace 10, with the exception of a minor amountof tempering air introduced into the primary air stream entering thepulverizers through the connections 41.

The firing rate of the furnace 10 is regulated by the operation of thepulverizers and the air flow to each row of burners. As shown, thepressure of the steam generated within the boiler is measured by aBourdon tube 60, with changes in pressure transmitted mechanically to anair pilot valve 61 Where such changes are transmitted as controlimpulses through a Standatrol 62 and ratio controller 63 to the powerpistons 64 and 65. The power pistons 64 are mechanically arranged toposition the valves in the ducts 38. Thus, the piston 64A positions thevalves in duct 38A, the piston 648 positions the v valves in duct 38B,and the piston 64C positions the valves in duct 38C. In a similar mannerthe power pistons 65A, 65B and 65C are mechanically arranged to positionthe valves 42A, 42B and 42C, respectively, of the pulverizers. It willbe appreciated that the flow of fuel to the burners is regulated inproportion to the flow of primary air to the pulverizers. As is wellknown in the art, this is accomplished by a diiferential ratio controlactuated by a measurement of the air flow to the pulverizer comparedwith a measurement of the air flow through the pulverizer. It will alsobe understood that the power pistons 65, or their equivalent can also beused to regulate the opening of a valve to control the flow of a liquidor gaseous fuel to the burners 30. The control impulse from the airpilot valve 61 is also transmitted, as a modifying element, to theaveraging relay 52 of the forced draft fan control.

A separate control system is interposed to regulate the operation of thepulverizer 31A and the burners 30A over a selected range of steamgenerating and superheating unit capacities. This system includes acontroller 66 actuated by a measurement of the steam flow leaving thegenerating unit. The control impulse from the controller 66 is passed toa pilot valve 67, which may be cam actuated to obtain desirable controlcharacteristics, and thence to a ratio controller 68 and an averagingrelay 70 where it is joined by the control impulse originating from theBourdon tube 60, after passing through a ca1ibrating relay 71. Thecontrol impulse thereafter is transmitted to the power pistons 64A and65A.

In the operation of the controls, the steam flow-air flow controller 48regulates the operation of the forced draft fan 32 through thepositioning of piston 47, and valves 45 and 46 so as to supply theproper amount of total air to the furnace 10, as required for thecombustion of sufficient fuel to generate the steam discharged from theboiler. This control system is modified by a superimposed controlirnpulse produced by the Bourdon tube 60 and Standatrol 62 so as toinsure immediate control respouse to a change in boiler operatingconditions. The change in boiler operating conditions created by achange in steam pressure actuates the ratio controller 63, while achange in boiler operating conditions created by a change in steam flowactuates the ratio controller 68. The steam pressure actuated ratiocontroller 63 creates a change in the position of the primary aircontrol dampers of pulverizers 31B and 31C, as well as the correspondingsecondary air dampers for the burners 30B and WC. However, in accordancewith the method of the present invention, the control impulse from thecontroller 63 will be ineffective over a selected operating capacityrange of the steam generating unit, in altering the damper positions ofpulverizer air control valve 42A and secondary air flow to the burners30A. This is accomplished by adjustment of the calibrating relay 71 andthe averaging relay '79. When the steam flow rate from the boiler unitis below a predetermined value, the pulverizer and burner power pistons64A and 65A are regulated in parallel with pistons 64B, 64C, 65B and 65Cin accordance with the control impulses from the ratio controller 63.Thus, the burners SilA are supplied with a substantially uniform supplyof fuel and combustion air in an upper range of steam generating andsuperheating capacity, while the burners 39B and 30C are operated inparallel at rates necessary to maintain the steam pressure requirementsof the unit. Below the selected range of unit output capacity, theburners 30A, 3M3 and 39C are operated in parallel in response to steampressure through the controller 63. To avoid an abrupt changeover in theoperation of the burners 39A at the lower end of their operation at asubstantially uniform fuel input, the relays 70 and 71 are adjusted topermit a gradual or smooth changeover from a predominantly steam flowregulation to a predominantly steam pressure regulation of the fuel andair supplied through the burners 36A. If desired, a cam actuated pilot67 can be used to accentuate or diminish the controlling impulseobtained from steam flow. In accordance with the usual control practice,selector valves 72 are provided in the control circuits to permitselective manual or automatic operation of the control elements. a

By way of example, Fig. 4 diagrammatically illustrates the operation ofa steam generating and superheating unit of the type shown in Fig. l, inaccordance with the present invention and the effect of this operationon the superheated steam temperature. As shown in the lower portion ofFig. 4, full load on the steam generating unit is carried by theoperation of all three rows of burners, with each row supplyingapproximately one-third of the total fuel requirement of this load.Also, at maximum capacity of the steam generating unit, each row ofburners is operated substantially at its full design capacity. As thecapacity of the unit is reduced, the upper row of burners (36A)continues to supply substantially its rated capacity of fuel, while thereduction in furnace fuel requirements is obtained by a reduction in theamount of fuel delivered by the lower and middle rows of burners (StlCand 308). For example, at 70% capacity of the steam generating unit, theburner row StlA continues to supply substantially its full ratedcapacity of fuel, while the remaining fuel requirements are supplied bythe burner rows MP3 and 3liC. At this steam output capacity,approximately one-half of the total fuel is supplied by burner row 39A,while the burner rows 3633 and 30C each supply approximately one-fourthof the total fuel requirement. A further reduction in the steam outputcapacity requires a corresponding reduction in the fuel delivered to thefurnace by the burner rows SllB and 30C, until the fuel input to thoseburners reaches a minimum preferred capacity. This condition is shown inFig. 4, at approximately 55% of boiler load, where one row of burners,namely 30C, is removed from service and the input from the burners 30Bis correspondingly increased. Between a boiler load of approximately 55and 45 percent, fuel is supplied to the furnace 10 at the same base rateby burners 30A, while burners 30B are operated at a rate sufficient tomake up the dilference in fuel requirements. Below 45% of boiler loadburner rows 30A and 30B are operated in parallel with each providingapproximately an equal share of the fuel requirements. The transition toparallel operation of burner rows 30A and 30B is illustrated by thecurved line separating the representation of upper and middle (orinterme diate) burner inputs occurring between approximately 45 and 50percent of boiler load. At 20 percent of boiler load, burner row 39B isremoved from service and all of the fuel requirements are supplied byburnerrow 30A, to its minimum operating capacity. The removal of anypulverizer from service, or its restarting, is accomplished manually inaccordance with usual operating practice.

In the operations described and illustrated in Fig. 4, the minimumoperating capacity rate of each row of burners has been shown asillustrative of a furnace supplied with pulverized coal from a pluralityof direct firing pulverizers. The operating range of each pulverizer androw of burners supplied thereby is generally typical of a practicaloperating range with this type of equipment, where the range may belimited by burner velocity considerations, flame stability or pulverizeroperations. Other minimum capacity values may be used, whether thesource of fuel may be pulverized coal from a storage or unit system,liquid or gaseous.

The upper portion of Fig. 4 illustrates the effect upon steam superheattemperatures with furnace fuel deliveries according to the presentinvention as compared with superheat temperatures when operating allburners in parallel. As shown, the dotted line illustrates superheattemperatures when the controlled firing technique of the presentinvention is utilized, while the solid line represents superheattemperatures, in the same unit, when the burners are operated inparallel according to furnace fuel requirements, as in the usualbalanced mode of burner operations. The dotted temperature line isconsiderably flatter than the solid line, and while both lines havesubstantially the same temperature value at rated boiler load, i. e.capacity, the dotted line crosses the bypass temperature control line atapproximately 45% of boiler load. Thus, burner operation according tothe present invention permits the maintenance of substantially uniformsuperheat output temperature from the unit between a load of 45% to 100%while using a conventional, and economical, gas by-pass arrangement forsuperheat temperature control. In addition, the drop in superheattemperatures at ratings below 45% of boiler load is considerably lessthan with conventional firing methods.

It will be noted the present invention provides for the operation of asteam generating unit through a wide range of operating capacities whilemaintaining a substantially uniform temperature of the superheat-edsteam produced. This is accomplished by the provision of a conventionalsuperheater gas by-pass, or alternately with a steam attemperator, inconjunction with a novel regulation of the fuel input to the boilerfurnace. Fuel is delivered at vertically spaced positions into avertically elongated furnace enclosed by fluid cooled walls and having aconvection type superheater positioned closely adjacent or within anupper furnace outlet. The fuel delivered to the upper portion of thefurnace is maintained at a substantially uniform input rate throughout amajor portion of the upper load range of the steam generating unit,while variations in fuel input to the furnace are obtained by changes inthe amount of fuel delivered to the lower portion of the furnace. Inthis manner the wall cooling efiect of the lower portion of the furnacewill have less effect upon the temperature of the combustion gasesleaving the furnace outlet and passing to the superheat elements.

The invention has been illustrated by the use of pulverized coal as afuel in generating and superheating steam, and a simple and efficientcontrol system for attaining the superheat temperature control of theinvention has been shown. However, it will be understood that theobjectives of the present invention can be attained by manual regulationof furnace fuel inputs, and that liquid or gaseous fluids can also beused to accomplish the advantageous superheat temperature regulation ofthe invention.

While in accordance with the provisions of the statutes we haveillustrated and described herein the best form of the invention nowknown to us, those skilled in the art will understand that changes maybe made in the form of the apparatus disclosed without departing fromthe spirit of the invention covered by our claims, and that Y certainfeatures of our invention may sometimes be used to advantage without acorresponding use of other features.

We claim:

l. In a vapor generating and superheating unit of the into theintermediate portion of said zone throughout a selected upper range ofvapor generating capacity requirement while varying the fuel and airintroduced into said opposite end portion in accordance with a change ofvapor generating capacity requirement, and directly varying the fuel andair introduced to both of said opposite end and intermediate portions ofsaid combustion zone equally in accordance with vapor generatingrequirements below said selected upper range of vapor generatingcapacity.

2. The method of maintaining a substantially uniform superheated vaportemperature over an extended operating capacity range of a vaporgenerator having an elongated fluid-cooled furnace with a heating gasoutlet in one end portion thereof and convection vapor superheaterelements positioned beyond said fluid-cooled furnace outlet in the pathof heating gas flow which comprises, introducing fuel and air forcombustion into the opposite end and intermediate portions of saidfurnace, said introduction of combustion fuel and air comprisingintroducing a substantially uniform flow of fuel and air into theintermediate portion of said furnace throughout a selected upper rangeof vapor generating rates While varying the fuel and air introduced intosaid opposite end portion in accordance with the actual change of vaporgenerating rates, directly varying the fuel and air introduced to bothof said opposite end and intermediate portions of said furnace equallyin accordance with vapor generating rates below said selected upperrange of vapor generating rates, and by-passiug at least a portion ofthe heating gases leaving said furnace around a portion of said vaporsuperheater elements at the upper vapor generating rates to lower vaporsuperheat temperatures.

3. The method of maintaining a substantially uniform superheated vaportemperature over an extended operating capacity range of a vaporgenerator having an elongated fluid-cooled furnace with a heating gasoutlet in one end portion thereof and convection vapor superheaterelements positioned beyond said fluid-cooled furnace outlet in the pathof heating gas flow which comprises, separately introducing air and fuelfor combustion into the opposite end and intermediate portions of saidfurnace, said introduction of combustion fuel and air comprisingintroducing a substantially uniform flow of fuel and air into theintermediate portion of said furnace throughout a selected upper rangeof vapor generating capacity while varying the fuel and air introducedinto said opposite end portion in accordance with a change of vaporpressure, directly varying the fuel and air introduced to both of saidopposite end and intermediate portions of said furnace equally inaccordance with generated vapor pressure below said selected upper rangeof vapor generating capacity, and attemperating said superheated vaporto maintain a substantially uniform temperature over at least a portionof the capacity range of said substantially uniform fuel and airintroduced into the intermediate portion of said furnace.

4. The method of regulating superheated vapor temperature over anextended operating capacity range of a vapor generator having anelongated fluid-cooled furnace with a heating gas outlet in one endportion thereof and a convection vapor superheater positioned beyondsaid fluidcooled furnace outlet in the path of the heating gas flowwhich comprises, introducing fuel and air for combustion into theopposite end and intermediate portions of said furnace, saidintroduction of combustion fuel and air comprising introducing asubstantially uniform flow of fuel and air into the intermediate portionof said furnace throughout a selected upper range of vapor generatingcapacity while varying the fuel and air introduced into said oppositefurnace end portion in accordance with a change of vapor generating loadas measured by a variation in vapor pressure, directly varying the fueland air introduced to both of said opposite end and intermediateportions of said furnace equally in accordance with vapor pressure belowsaid selected upper range of vapor generating load, in a manner toobtain a substantially uniform ratio of air and fuel delivered to bothportions of said furnace.

5. Apparatus for generating and superheating vapor comprising, incombination, walls lined by spaced vapor generating tubes defining anupright elongated furnace having a heating gas outlet in one portionthereof, a convection superheater positioned adjacent said furnace gasoutlet in the path of gas flow leaving said furnace, a plurality offixed position fuel burners opening to said furnace and spaced atdifferent positions from said furnace outlet, means for delivering asubstantially uinform fuel and air input to a portion of said furnace inan upper range of vapor generating flow rates While varying the fuel andair input to another portion of said furnace in accordance with theactual pressure requirement of the vapor generating rate, and means fordelivering substantially equal amounts of fuel and air into bothportions of said furnace at required vapor generating rates less thanthe lower limit of said upper range of required vapor generating rates.

6. Apparatus for generating and superheating vapor comprising, incombination, walls lined by spaced vapor generating tubes defining anupright elongated furnace having a heating gas outlet in one portionthereof, a convection superheater positioned adjacent said furnace gasoutlet in the path of gas flow leaving said furnace, a plurality of rowsof fuel burners opening to said furnace with each burner row spaced at adifferent position from said furnace outlet, a controllable source offuel for each row of burners, a controllable source of preheatedsecondary combustion air for each row of burners, control meansresponsive to vapor pressure for regulating the amount of fuel and airdelivered to each row of burners, and a separate control meansresponsive to vapor flow for maintaining a substantially uniformdelivery of fuel and secondary air to the row of burners closest to saidsuperheater throughout an upper range of vapor generating rates, saidseparate control means overriding said vapor pressure responsive controlmeans in said upper range of vapor generating rates and becomingineffective below said upper rate range.

7. Apparatus for generating and superheating vapor comprising, incombination, walls lined by spaced vapor generating tubes defining avertically elongated furnace having a heating gas outlet in the upperportion thereof, a convection superheater positioned adjacent saidfurnace gas outlet in the path of gas flow leaving said furnace, aplurality of rows of fuel burners opening to said furnace and verticallyspaced at difierent positions from said furnace outlet, a pulverizerconnected with each row of said fuel burners, a control responsive tovapor pressure for regulating the fuel input to each row of said burnersfrom each pulverizer in accordance with the output rating of said vaporgenerator, and a separate control responsive to vapor flow formaintaining a substantially uniform fuel input to the uppermost row ofburners from the connected pulverizer to said furnace throughout anupper range of vapor generating rates, said separate control overridingsaid vapor pressure responsive control in said upper range of vaporgenerating rates and becoming inefiective below said upper rate range,and means for maintaining a substantially uniform ratio of secondarycombustion air and pulverized fuel delivered to each row of saidburners.

References Cited in the file of this patent UNITED STATES PATENTS1,098,935 Brown June 2, 1914 10 Brown May 4, 1920 Jackson j Nov. 23,1937 Blizard July 12, 1938 Wunsch et al Mar. 7, 1939 Bailey Apr. 9, 1940Bailey et al. Feb. 18, 1941 Mayo JunelO, 1941 Koch Sept. 2, 1941Kreisinger et al Nov. 28, 1944 Blizard Jan. 16, 1945 Nagel Dec. 11, 1945Jacobs July 4, 1950 Woolley May 15, 1951 Mittendorf Nov. 20, 1951Lacerenz-a Mar. 25, 1952 Frisch Apr. 22, 1952 FOREIGN PATENTS FranceJan. 7, 1949

